Hydraulic system



March 22, 1966 I A; ARAZIJA 3,241,452

HYDRAULIC SYSTEM Filed Feb. 17, 1964 United States Patent 3,241,462 HYDRAULlC SYSTEM Arvin Karazija, Chicago, IlL, assignor to Allis-Chalmers Manufacturing Company, Milwaukee, Wis. Filed Feb. 17, 1964, Ser. No. 345,186 5 Claims. (Cl. 91-443) This invention relates to an improved hydraulic system for a lift cylinder and particularly to a system wherein a dumping valve assembly is interposed between the lift cylinder and the control valve to bypass fluid to the reservoir when the control valve is in a lower position.

In litt trucks it is oiten desirable to rapidly lower the lift truck carriage to shorten the work cycle. By utilizing means to rapidly expel the fluid from the lift cylinder, upon the control valve being moved to a lower position, it is possible to achieve rapid lowering even though a relatively small control valve is used. My invention lends itself to field modification of existing lift trucks by inexpensive additions and alterations of the lift truck hydraulic system.

It is an object of this invention to provide a dumping valve in a hydraulic circuit for the lift cylinder whereby the fluid from the lift cylinder is bypassed to the reservoir without passing through the control valve, when the control valve is shifted to a lower position.

It is a further object of this invention to provide an improved hydraulic system for a lift truck whereby the lowering speed of a lift cylinder is increased.

It is an object of this invention to provide a hydraulic system for a lift cylinder of a lift truck wherein a dumping valve is employed intermediate the control valve in the lift cylinder which automatically assumes a fluid dumping position when the control valve is in a lower position.

These and other objects and advantages of this invention will be apparent upon reading the following description in conjunction with the drawings in which:

FIG. 1 is a schematic view of a hydraulic control system of this invention used in a lift truck; and

FIG. 2 is a schematic view of the hydraulic system of this invention showing the dumping valve in a fluid dump ing position.

Referring to FIG. 1, the lift truck 11 is provided with an operators seat 12, a steering wheel 13 and litt cylinder control valve lever 14. The lift truck 11 also includes a conventional mast assembly 16 on which a load sup porting carriage 17 is mounted for reciprocal vertical movement by a lift cylinder 18. The lift truck engine 21 drives a pump 22, shown schematically, through a drive train, not shown. The pump 22 draws fluid from a reservoir 23 and delivers the fluid to a control valve 24 through supply line 26. A dump-ing valve assembly 3 1 and a flow regulator 32 are connected in series in the lift cylinder supply line 33. Return to reservoir lines 36, 37 are connected to the control valve 24 and to the housing 40 of dumping valve assembly 31, respectively. The dumping valve spool 43- assumes the fluid blocking position, shown in FIG. 1, when the control valve 24 is in a hold position, and in this position of the control valve the fluid from pump 22 passes through the control valve 24 and back to reservoir through return to reservoir conduit 36, lift cylinder supply line 33 is blocked at the control valve 24, and the pressure at the inlet port 41 of the dumping valve assembly 3 1 is the same as the pressure at the outlet port 42. Thus the pressure on opposite ends of the shifta'ble dumping valve or spool 43 is equalized. The spool 43 is in fluid tight sealing relation with the cylindrical bore 46 in dumping valve housing 40, prevent ing flow through bypass passage 61 or restricted flow rate passage 66. The spool 43 is biased to its closed position illustrated in FIG. 1 by a suitable coil spring 47.

Land 51 is spaced from land 52 of the plunger by a reduced diameter portion 53. A bleed passage 54 insures equalization of pressure on opposite sides of the spool 43 when the control valve is in its old position.

When the control valve 24 is shifted to a raise positi-on, fluid is delivered through conduit 33 to port 4 1, thence passes through check valve means 56, which includes a ball 57 and a biasing spring 58, thence through passage 59 to delivery port 42 through flow regulator 32 and then to cylinder 18. in the raise position Otf the control valve, the fluid pressure on opposite ends of the valve 43, that is in chambers 44 and 45, is approximately equal and the spring 47 will maintain the spool 43 in its closed or blocking position, as illustrated in FIG. 1.

When the control valve 24 is moved to its lower position, as illustrated in FIG. 2, the fluid delivered by the engine driven pump 22 returns to reservoir through conduit 36 and fluid from the lift cylinder 18 flows through the conventional flow regulator 32 thence to the dumping valve assembly 31. Check valve 56 prevents return flow through passage 59 and bleed passage 54 permits only a slight amount of flow, thus permitting the pressure in chamber 45 to drop to near zero. The pressure in chamber 44 will increase sufficiently, due to the weight of carriage 17 imposed on cylinder 18, to move spool 43 to the position illustrated in FIG. 2, wherein the bypass passage 61 to reservoir port 62 is open and fluid returns to reservoir through bypass conduit 37 forming a continuation of bypass passage 61. The dumping valve assembly 31 is designed to provide a high bypass flow rate thus permitting high speed lowering of the carriage. This bypass flow rate through the dumping valve to reservoir is substantially greater than that which might otherwise be permitted through the conventional control valve 24, if the dumping valve assembly 31 were omitted from the circuit. The flow regulator valve 32 is not essential to the operation of the dumping valve, however, it is desired for many lift truck operations. In the usual lowering sequence, the lift cylinder 18 is contracted until the carriage 17 is approximately a foot off the ground and then the control valve 24 will be moved by the operator through control lever 14, to hold position and when. this occurs the line 33 would be blocked at the control valve and bleed passage 54 will permit equalization of pressure on op posite ends of spool 43 and the spring 47 will return the dumping valve plunger 43 to the fiuid blocking position shown in FIG. 1. If it is desired to lower the carriage slowly the control valve is throttled to permit a build-up of pressure in chamber 45, through bleed passage 54, thereby partially closing dumping valve spool 43. From the foregoing description it is apparent that a novel hydraulic system for a lift cylinder has been provided wherein check valve means 56 is placed in the fluid passage means 33 between the control valve 24 and lift cylinder 18, a bypass passage connects the reservoir to the passage means 33 intermediate the check valve means 56 and the cylinder 18, and a shiftable dumping valve 43 is biased by spring 47 to a closed position blocking the bypass passage and is shiftable to an open position when the fluid pressure on the downstream side of the check valve exceeds the pressure on the upstream side thereof by a predetermined value. A relatively small control valve can be employed in a hydraulic systeiii when the dumping valve of this invention is used. The lowering speed is faster than would be afforded by returning the fluid to reservoir through the control valve. Thus several sizes of lift trucks can use the same size control valve when the dumping valve of this invention is employed. This permits standardizing on a single control valve for a number of lift truck sizes with the resulting lower expense of procurement and stocking of service parts. The check valve 56 and dumping valve 43 are built into the same housing 40 thereby providing an assembly 31 which is easily installed during manufacture of the truck or which may be sold as a modification kit.

The embodiments of the invention for which an exclusive property or privilege is claimed are defined as follows:

1. A hydraulic system for a lift cylinder of a lift truck comprising:

a hydraulic reservoir,

a pump connected to draw fluid from said reservoir,

a control valve operatively connected to said pump and reservoir and having raise, hold and lower position, and

a dumping valve including a housing,

a first port in said housing connected in fluid receiving relation with said control valve,

a second port in said housing connected in fluid supplying and exhausting relation to said cylinder,

a third port in said housing connected in fluid dumping relation to said reservoir,

a first passage in said housing between said first and second ports,

a check valve in said first passage,

a second passage in said housing between said third port and said first passage on the outlet side of said check valve,

a bore in said housing intersecting said second passage having one end in free fiuid communica- 4 tion with said first passage on the inlet side of said check valve, passage means placing the other end of said bore in fluid communication with said second port,

a valve spool in said bore having a pair of fluid blocking lands in sealing engagement with said bore and a portion intermediate said lands, and

means biasing said spool to a fluid blocking position in which fluid flow through said second passage is prevented, said spool being shifted to an open position in which fluid flow through said second passage is permitted when the fluid pressure at said second port exceeds the fluid pressure at said first port by a predetermined value.

2. The structure set forth in claim 1 wherein said passage means has a restricted flow rate to act as a surge dampener for said other end of said bore.

3. The structure set forth in claim 1 and further comprising bleed means for permitting a low rate of flow from said second port to said first port.

4. The structure set forth in claim 3 wherein said bleed means is a bleed passage in said housing.

5. The structure set forth in claim 4 wherein said passage means has a sufiiciently low flow rate to act as a surge dampener for said other end of said bore.

References Cited by the Examiner UNITED STATES PATENTS 3,138,676 6/1964 Gratzmuller 9l443 FOREIGN PATENTS 734,899 4/1943 Germany.

SAMUEL LEVINE, Primary Examiner. 

1. A HYDRAULIC SYSTEM FOR A LIFT CYLINDER OF A LIFT TRUCK COMPRISING: A HYDRAULIC RESERVOIR, A PUMP CONNECTED TO DRAW FLUID FROM SAID RESERVOIR, A CONTROL VALVE OPERATIVELY CONNECTED TO SAID PUMP AND RESERVOIR AND HAVING RAISE, HOLD AND LOWER POSITION, AND A DUMPING VALVE INCLUDING A HOUSING, A FIRST PORT IN SAID HOUSING CONNECTED IN FLUID RECEIVING RELATION WITH SAID CONTROL VALVE, A SECOND PORT IN SAID HOUSING CONNECTED IN FLUID SUPPLYING AND EXHAUSTING RELATION TO SAID CYLINDER, A THIRD PORT IN SAID HOUSING CONNECTED IN FLUID DUMPING RELATION TO SAID RESERVOIR, A FIRST PASSAGE IN SAID HOUSING BETWEEN SAID FIRST AND SECOND PORTS, A CHECK VALVE IN SAID FIRST PASSAGE, A SECOND PASSAGE IN SAID HOUSING BETWEEN SAID THIRD PORT AND SAID FIRST PASSAGE ON THE OUTLET SIDE OF SAID CHECK VALVE, A BORE IN SAID HOUSING INTERSECTING SAID SECOND PASSAGE HAVING ONE END IN FREE FLUID COMMUNICATION WITH SAID FIRST PASSAGE ON THE INLET SIDE OF SAID CHECK VALVE, PASSAGE MEANS PLACING THE OTHER END OF SAID BORE IN FLUID COMMUNICATION WITH SAID SECOND PORT, A VALVE SPOOL IN SAID BORE HAVING A PAIR OF FLUID BLOCKING LANDS IN SEALING ENGAGEMENT WITH SAID BORE AND A PORTION INTERMEDIATE SAID LANDS, AND MEANS BIASING SAID SPOOL TO A FLUID BLOCKING POSITION IN WHICH FLUID FLOW THROUGH SAID SECOND PASSAGE IS PREVENTED, SAID SPOOL BEING SHIFTED TO AN OPEN POSITION IN WHICH FLUID FLOW THROUGH SAID SECOND PASSAGE IS PERMITTED WHEN THE FLUID PRESSUER AT SAID SECOND PORT EXCEEDS THE FLUID PRESSURE AT SAID FIRST PORT BY A PREDETERMINED VALUE. 